1. Field of the Invention
This invention relates to a map displaying apparatus, and particularly to a map displaying apparatus which can display a map on a screen with facilities superimposed on the map.
2. Description of the Related Art
A car navigation system is installed in a car and detects the position of the car as the car travels by means of GPS or the like and displays that position on a display together with a road map to help the driver of the car reach a destination smoothly. Navigation systems which not only show the present position of the car but also display marks representing certain facilities such as, for example, gas stations and restaurants are also known, and these are effective when a driver wants to go to one of these facilities. Specifically, when a driver selects a category of facility to be displayed, for each facility of that category located in the region covered by the map on which the present position of the car is being displayed, the navigation system displays a facility mark M denoting that facility in the appropriate position on the map.
In the display example shown in FIG. 4A, there are only a few facility marks M. Therefore, it is possible to distinguish easily with roads as references the positions in which the facilities are located. However, such facilities are not distributed evenly in all areas, and the density in which facilities exist is different in different areas; for example, facilities of the same category may exist in concentration in urban areas while being only found here and there in country areas. Consequently, for example in the display example shown in FIG. 4B, although the scale of the map is the same as that of FIG. 4A, because there is a high concentration of facilities of the selected category, there are many facility marks in the map region being displayed and the roads themselves are hidden. When the roads are hidden, it is impossible for the route to a chosen facility to be distinguished, and the benefit of the facility guidance is lost. When, on the other hand, facilities of the selected category are few and far between, it may happen that no facility mark is displayed in the map region being displayed. In this case also the benefit of the facility guidance is lost.
Therefore, when as in the display example shown in FIG. 4B there are too many facility marks, it is necessary for the scale of the displayed map to be increased to obtain a suitable density of facility marks, for example as shown in FIG. 4C. (The scale of the displayed map shown in FIG. 4C is four times the scale of the displayed map shown in FIG. 4B.) When, on the other hand, no facility marks whatsoever are displayed in the map region being displayed, reversely it is necessary for the scale of the displayed map to be reduced until there is at least one facility mark in the map region being displayed. The user himself or herself has had to understand these facility mark display states and perform an operation to change the scale to one which the user finds easy to read, and this has been extremely troublesome.
Also, in a navigation system, when a user is specifying a facility, there are times when although the user knows the category of the facility (for example Airport, Bank, Gas Station or Park) and perhaps even the individual name of the facility (for example XX Airport, XY Branch of XZ Bank, YX Gas Station or YY Park or the like), because the user is unfamiliar with the geography of the vicinity in which the facility is located, he or she is unable to locate the position of the facility on a map. To make it possible for a user to specify a facility even in this kind of case, data search systems capable of searching for data indicating the position of a facility on the map from the category and/or the individual name of the facility are in use.
However, when a user specifies the category of a facility, the data search system finds all facilities of that category that have been preregistered in the system. Here, when many facilities of the same category have been preregistered, if all the facilities found are displayed randomly on the display, the user looking at that display may be at a loss to decide which facility to choose.
To avoid this, some data search systems in the related art have a function of, when many facilities of the same category have been preregistered, rearranging the many facilities found in the order of their proximity to the present position and displaying these rearranged facilities on the display in the form of a list. With this function, because facilities close to the present position can be ascertained just by looking at the display, it is easy for the user to choose a facility. The processing of rearranging the facilities found in a search in the order of their proximity to the present position is carried out by a microcomputer.
However, when many facilities found in a search are being rearranged in the order of their proximity to a present position, the more facilities there are the longer the processing time required for their rearrangement becomes. When an ordinary sorting algorithm is used, the processing time required for the rearrangement increases exponentially with respect to the number of facilities (the amount of data) found in the search.
As the waiting time from when a user specifies a category of facility to when facilities of that category found in a search are displayed on a display, normally, although there are differences among individuals, the limit that a user will tolerate is said to be about two to three seconds. However, when many facilities are found in the search, it takes a great deal of time from the user specifying the facility category to the facilities of that category found in the search being displayed on the display, and there is a risk of this time exceeding the limit that the user will tolerate.